When a medical device is placed in the body, or in contact with body fluids, a number of different reactions are set into motion, some of them resulting in the coagulation of the blood in contact with the device surface. In order to counteract this serious adverse effect, the well-known anti-coagulant compound heparin has for a long time been administered systemically to patients before the medical device is placed in their body, or when it is in contact with their body fluids, in order to provide an antithrombotic effect.
Thrombin is one of several coagulation factors, all of which work together to result in the formation of thrombi at a surface in contact with the blood. Antithrombin (also known as antithrombin III) (“AT” or “ATIII”) is the most prominent coagulation inhibitor. It neutralizes the action of thrombin and other coagulation factors and thus restricts or limits blood coagulation. Heparin dramatically enhances the rate at which antithrombin inhibits coagulation factors.
However, systemic treatment with high doses of heparin is often associated with serious side-effects of which bleeding is the predominant. Another rare, but serious complication of heparin therapy is the development of an immunological response called heparin induced thrombocytopenia (HIT) that may lead to thrombosis (both venous and arterial). High dose systemic heparin treatment e.g. during surgery also requires frequent monitoring of the activated clotting time (used to monitor and guide heparin therapy) and the corresponding dose adjustments as necessary.
Therefore solutions have been sought where the need for a systemic heparinisation of the patient would be unnecessary or can be limited. It was thought that this could be achieved through a surface modification of the medical devices using the anti-coagulant properties of heparin. Thus a number of more or less successful technologies have been developed where a layer of heparin is attached to the surface of the medical device seeking thereby to render the surface non-thrombogenic. For devices where long term bioactivity is required, the heparin layer should desirably be resistant to leaching and degradation.
Heparin is a polysaccharide carrying negatively charged sulfate and carboxylic acid groups on the saccharide units. Ionic binding of heparin to polycationic surfaces has been attempted, but these surface modifications tended to suffer from lack of stability over time resulting in lack of non-thrombogenic function, as the heparin leached from the surface.
Thereafter different surface modifications have been prepared wherein the heparin has been covalently bound to groups on the surface.
One of the most successful processes for rendering a medical device non-thrombogenic has been the covalent binding of a heparin fragment to a modified surface of the device. The general method and improvements thereof are described in European patents: EP-B-0086186, EP-B-0086187, EP-B-0495820 and U.S. Pat. No. 6,461,665.
These patents describe the preparation of surface modified substrates by first, a selective cleavage of the heparin polysaccharide chain, e.g. using nitrous acid degradation, leading to the formation of terminal aldehyde groups. Secondly, the introduction of one or more surface modifying layers carrying primary amino groups on the surface of the medical device, and thereafter reacting the aldehyde groups on the polysaccharide chain with the amino groups on the surface modifying layers followed by a reduction of the intermediate Schiff's bases to form stable secondary amine bonds.
Other methods of modifying surfaces are known. For example, US 2005/0059068 relates to a substrate for use in microassays. An activated polyamine dendrimer is covalently bonded to the surface of the substrate through a silane containing moiety. The dendrimer has branch points which are tertiary amines and terminal residues which are NH2, OH, COOH or SH groups. Molecules containing OH or NH2 functional groups can be bound to the dendrimer via the terminal residues of the dendrimer. Since the substrate is for use in microassays, it is usually a slide, bead, well plate, membrane etc. and the moiety containing the OH or NH2 group is a nucleic acid, protein or peptide.
WO 03/057270 describes a device, for example a contact lens, with a lubricious coating having high surface hydrophilicity. A number of examples of coating materials are given including glycosaminoglycans (e.g. heparin or chondroitin sulfate) and PAMAM dendrimers. PAMAM dendrimers are said to be among the preferred coatings. The document exemplifies a contact lens having multiple layers of PAMAM dendrimer and polyacrylamide-co-polyacrylic acid copolymer (PAAm-co-PAA). The coating is formed by consecutively dipping the contact lens into solutions of the two coating materials, with the outer layer being PAAm-co-PAA.
US 2003/0135195 teaches a medical device such as a catheter with a highly lubricious hydrophilic coating formed from a mixture of colloidal aliphatic polyurethane polymer, an aqueous dilution of poly(1-vinylpyrrolidone-co-2-dimethylaminoethylmethacrylate)-PVP and dendrimers. The document teaches that the coating may be applied to the device by dipping the device in a colloidal dispersion of the aliphatic polyurethane polymer in a solution of poly(1-vinylpyrrolidone-co-2-dimethylaminoethylmethacrylate)-PVP and an active agent (e.g. heparin) in a mixture of dendrimer, water, N-methyl-2-pyrrolidone and triethylamine. The document teaches that heparin may be contained in the voids within the dendrimers. The document also teaches that the loaded heparin will elute from the hydrophilic polymer matrix at a predetermined rate.
US 2009/0274737 teaches implants such as stents having a hydrophilic surface with a wetting angle of ≦800. There may be one, two or more anti-coagulant ingredients permanently bound to the surface and examples of anticoagulants include heparin and certain dendrimers, especially sulphated dendrimers. The surface may be functionalised in order to bind the anticoagulant and examples of functionalization are silanization and reaction with 1,1′-carbonyldiimidazole (CDI).
U.S. Pat. No. 4,944,767 relates to a polymeric material which is able to adsorb high quantities of heparin. The material is a block copolymer in which polyurethane chains are interconnected with polyamidoamine chains.
Our earlier application WO 2010/029189 relates to a medical device having a coating with an anticoagulant molecule such as heparin covalently attached to the coating via a 1,2,3-triazole linkage. The document describes the azide or alkyne functionalisation of a polyamine; the preparation of alkyne or azide functionalised heparin (both native and nitrous acid degraded heparin); and the reaction to link the derivatised heparin to the derivatised polymer via a 1,2,3-triazole linker.
The product described in WO 2010/029189 has many advantages but we have sought to develop an improved material in which the bioavailability of the heparin or other attached anti-coagulant molecule is increased, which may have greater stability on aging and which can be manufactured by a process which is robust and produces a product of high consistency. Heparins have the ability to bind a wide variety of biomolecules including enzymes, serine protease inhibitors (such as antithrombin), growth factors and extracellular matrix proteins, DNA modification enzymes and hormone receptors. If used in chromatography, heparin is not only an affinity ligand but also an ion exchanger with high charge density. Thus biomolecules can be specifically and reversibly adsorbed by heparins immobilized on an insoluble support. Immobilised heparins therefore have a number of useful non-medical applications, particularly for analysis and separation.